That explains it then, it is Smoke that is creating the .patch files. As suggested, I'll modify my master script to create a .patch file in the p4 sync'd clone with the correct number in it, then it should all work OK.
I now have this:
$ more */.patch :::::::::::::: devel-5.10/.patch :::::::::::::: 22084 :::::::::::::: maint-5.6/.patch :::::::::::::: 21880 :::::::::::::: maint-5.8/.patch :::::::::::::: 22090
-- Alan Burlison --
